There are two inflation technologies currently used for air bags. The most commonly used technology is sodium azide-based pyrotechnic. The other technology is called a hybrid because it utilizes an inert compressed gas coupled with a pyrotechnic heater. Upon ignition, the pyrotechnic heater causes the stored gas to expand. This expanding gas fills the air bag cushion to provide occupant restraint. Currently, hybrid driver side air bag inflators are mainly made with a torroidal steel gas bottle and a separate chamber for the heater material.
Hybrid inflators are typically more desirable than a simple pyrotechnic inflator due to their lower cost. The most important problem in the driver side inflator is mass. Currently, most hybrid driver side inflators are over 250 grams heavier than conventional pyrotechnic inflators.
It is preferable that the pyrotechnic charge not be in pressure volume as the stored gas since the pyrotechnic material will take up space which would be more readily used for compressed gas. Secondly, pyrotechnic material held within a pressurized, typically inert gas is much harder to ignite.
Furthermore, it is also preferable that machined surfaces be avoided in the gas pressure volume since machine components must utilize material of a more expensive high purity type since cheaper materials which can be made into stampings often contain metal impurity runners which allow pressurized gas to leak through.